1. Field of the Invention
This invention relates broadly to lenses and polymeric materials useful in optic and ophthalmic arts. More specifically, this invention relates to polymeric materials and treatment processes useful in the manufacture of contact lenses. Still more specifically, this invention relates to contact lenses useful as extended-wear contact lenses.
2. Description of the Related Art
A wide variety of research has been conducted in the field of biocompatible polymers. The definition of the term "biocompatible" depends on the particular application for which the polymer is designed. In the field of ophthalmic lenses, and in particular in the field of contact lenses, a biocompatible lens may be generally defined as one which will not substantially damage the surrounding ocular tissue and ocular fluid during the time period of contact. The phrase "ophthalmically compatible" more appropriately describes the biocompatibility requirements of ophthalmic lenses.
One ophthalmic compatibility requirement for contact lenses is that the lens must allow oxygen to reach the cornea in an amount which is sufficient for long-term corneal health. The contact lens must allow oxygen from the surrounding air to reach the cornea because the cornea does not receive oxygen from the blood supply like other tissue. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation causes the undesirable growth of blood vessels in the cornea. "Soft" contact lenses conform closely to the shape of the eye, so oxygen cannot easily circumvent the lens. Thus, soft contact lenses must allow oxygen to diffuse through the lens to reach the cornea.
Another ophthalmic compatibility requirement for soft contact lenses is that the lens must not strongly adhere to the eye. Clearly, the consumer must be able to easily remove the lens from the eye for disinfecting, cleaning, or disposal. However, the lens must also be able to move on the eye in order to encourage tear flow between the lens and the eye. Tear flow between the lens and eye allows for debris, such as foreign particulates or dead epithelial cells, to be swept from beneath the lens and, ultimately, out of the tear fluid. Thus, a contact lens must not adhere to the eye so strongly that adequate movement of the lens on the eye is inhibited.
While there exist rigid gas permeable ("RGP") contact lenses which have high oxygen permeability and which move on the eye, RGP lenses are typically quite uncomfortable for the consumer. Thus, soft contact lenses are preferred by many consumers because of comfort. Moreover, a contact lens which may be continuously worn for a period of a day or more (including wear during periods of sleeping) requires comfort levels which exclude RGP lenses as popular extended-wear candidates.
In order to balance the ophthalmic compatibility and consumer comfort requirements in designing a daily wear soft contact lens, polymers and copolymers of 2-hydroxyethyl methacrylate (HEMA) were developed. These hydrophilic polymers move well on the eye and provide sufficient oxygen permeability for daily wear. Certain soft contact lenses have been approved by the FDA for extended wear periods of up to about 6 nights of overnight wear and seven days of daily wear. However, the consumer cannot safely and comfortably wear these poly(HEMA) lenses for extended periods of seven days or more, because the oxygen permeability is insufficient. True extended wear (i.e., seven days or more) of these lenses may result, at a minimum, in corneal swelling and development of surface blood vessels in the cornea.
In order to improve oxygen permeability, polymers containing silicone groups were developed. A variety of siloxane-containing polymers have been disclosed as having high oxygen permeability. For example, see U.S. Pat. Nos. 3,228,741; 3,341,490; 3,996,187; and 3,996,189. However, polysiloxanes are typically highly lipophilic. The properties (e.g., lipophilicity, glass transition temperature, mechanical properties) of known polysiloxanes has resulted in contact lenses which adhere to the eye, inhibiting the necessary lens movement. In addition, polysiloxane lipophilicity promotes adhesion to the lens of lipids and proteins in the tear fluid, causing a haze which interferes with vision through the lens.
There have been attempts to blend the desirable hydrophilic properties of hydrophilic polymers, formed from monomers such as HEMA, with the desirable oxygen permeability of polymers formed from siloxane-containing monomers. For example, see U.S. Pat. Nos. 3,808,178; 4,136,250; and 5,070,169. However, prior attempts at producing a true extended wear contact lens have been unsuccessful, either because of the effect of the extended-wear lens on corneal health or because the lens would not move on the eye. Thus, there remains a need for an ophthalmically compatible, transparent polymeric material which is suited to extended periods of continuous contact with ocular tissue and tear fluid.